1. Field of the Invention
The present invention relates to welded steel pipes suitable for forming structural components and underbody components of vehicles. In particular, the present invention relates to enhancement of hydroformability of welded steel pipes.
2. Description of the Related Art
Hollow structural components having various cross-sectional shapes are used in vehicles. Such hollow structural components are typically produced by spot welding parts formed by press working of a steel sheet. Since hollow structural components of current vehicles must have high shock absorbability for collision impact, the steels used as raw materials must have higher mechanical strength. Unfortunately, such high-strength steels exhibit poor press formability. Thus, it is difficult to produce structural components having highly precise shapes and sizes without defects from the high-strength steels by usual press forming.
A method that attempts to solve such a problem is hydroforming in which the interior of a steel pipe is filled with a high-pressure liquid to deform the steel pipe into a component having a desired shape. In this method, the cross-sectional size of the steel pipe is changed by a bulging process. A component having a complicated shape can be integrally formed and the formed component exhibits high mechanical strength and rigidity. Thus, the hydroforming attracts attention as an advanced forming process.
In the hydroforming process, electrically welded pipes composed of low or middle carbon steel sheets containing 0.10 to 0.20 mass percent carbon are often used due to high strength and law cost. Unfortunately, the electrically welded pipes composed of low or middle carbon steel sheets have poor hydroformability; hence, the pipes cannot be sufficiently expanded.
A countermeasure to enhance the hydroformability of the electrically welded pipes is use of ultra-low carbon steel sheet containing an extremely reduced amount of carbon. The electrically welded pipes composed of the ultra-low carbon steel sheet exhibit enhanced hydroformability. However, the seam of the pipe causes softening with grain growth by heat of seam welding during a pipe forming process, so that the seam is intensively deformed in a bulging process, thereby impairing the high ductility of the raw material. Thus, it is desired that welded pipes have enhanced mechanical properties and excellent seam properties durable for hydroforming.
An object of the present invention is to provide a welded steel pipe having excellent hydroformability durable for a severe hydroforming process.
Another object of the present invention is to provide a method for making the welded steel pipe.
In the present invention, the welded steel pipe has a tensile strength TS of about 780 MPa or more and a nxc3x97r product of the n-value and the r-value of about 0.15 or more. In a more preferred embodiment, the welded steel pipe has a tensile strength TS of in the range of about 780 MPa to about 980 MPa, and a nxc3x97r product of about 0.22 or more. In the preferred embodiment, preferably the n-value is at least 0.15 or the r-value is at least 1.5. In another more preferred embodiment, the welded steel pipe has a tensile strength TS of more than about 980 MPa, and a nxc3x97r product of about 0.15 or more. In this embodiment, preferably the n-value is at least 0.10 or the r-value is at least 1.0.
The inventors have intensively investigated compositions of welded steel pipes and methods for making the welded steel pipes in order to solve the above problems, and have discovered that a welded steel pipe that contained about 0.05 to about 0.3 mass percent carbon and a variable amount of Mn depending on the target properties and that was diameter-reduction-rolled at a accumulated diameter reduction rate of about 35% or more and a finish rolling temperature of about 500xc2x0 C. to about 900xc2x0 C. has a high nxc3x97r product (product of an n-value and an r-value) and exhibits excellent hydroformability.
The inventors have completed the present invention after additional investigation in view of the above results.
According to a first aspect of the present invention, a welded steel pipe having excellent hydroformability has a composition comprising, on the basis of mass percent, about 0.05% to about 0.3% C; about 0.01% to about 2.0% Si; more than about 1.5% to about 5.0% Mn; about 0.01% to about 0.1% P; about 0.01% or less of S; about 0.01% to about 0.1% Cr; about 0.01% to about 0.1% Al; about 0.01% to about 0.1% Nb; about 0.01% to about 0.3% Ti; about 0.001% to about 0.01% N; and the balance being Fe and incidental impurities, wherein the tensile strength of the welded steel pipe is about 780 MPa or more, and the nxc3x97r product of the n-value and the r-value is about 0.15 or more.
Preferably, the composition comprises, on the basis of mass percent, about 0.05% to about 0.3% C; about 0.01% to about 2.0% Si; more than about 1.5% to about 2.0% Mn; about 0.01% about 0.1% P; about 0.01% or less of S; about 0.01% to about 0.1% Cr; about 0.01% to about 0.1% Al; about 0.01% to about 0.1% Nb; about 0.1% to about 0.3% Ti; about 0.001% to about 0.01% N; and the balance being Fe and incidental impurities, wherein the tensile strength of the welded steel pipe is in the range of about 780 MPa to about 980 MPa, and the nxc3x97r product of the n-value and the r-value is about 0.22 or more.
Preferably, the n-value is about 0.15 or more or the r-value is about 1.5 or more.
Preferably, the composition further comprises at least one element group selected from the group consisting of group A and Group B, on the basis of mass percent, wherein Group A includes at least one element of about 1.0% or less of Cu, about 1.0% or less of Ni, about 1.0% or less of Mo, and about 0.01% or less of B; and Group B includes at least one element of about 0.02% or less of Ca and about 0.02% or less of a rare earth metal.
Alternatively, the composition preferably comprises, on the basis of mass percent, about 0.05% to about 0.3% C; about 0.01% to about 2.0% Si; more than about 2.0% to about 5.0% Mn; about 0.01% to about 0.1% P; about 0.01% or less of S; about 0.01% to about 0.1% Cr; about 0.01% to about 0.1% Al; about 0.01% to about 0.1% Nb; about 0.01% to about 0.1% Ti; about 0.001% to about 0.01% N; and the balance being Fe and incidental impurities, wherein the tensile strength of the welded steel pipe exceeds about 980 MPa, and the nxc3x97r product of the n-value and the r-value is about 0.15 or more.
In such a composition, preferably, the n-value is about 0.10 or more or the r-value is about 1.0 or more.
Preferably, such a composition further comprises at least one element group selected from the group consisting of Group A and Group B, on the basis of mass percent, wherein Group A includes at least one element of about 1.0% or less of Cu, about 1.0% or less of Ni, about 1.0% or less of Mo, and about 0.01% or less of B; and Group B includes at least one element of about 0.02% or less of Ca and about 0.02% or less of a rare earth metal.
According to a second aspect of the present invention, a method for producing a welded steel pipe having excellent hydroformability comprises: heating or soaking an untreated welded steel pipe having a steel composition containing, on the basis of mass percent: about 0.05% to about 0.3% C, about 2.0% or less of Si, more than about 1.5% to about 5.0% Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.1% or less of Cr, about 0.1% or less of Al, about 0.1% or less of Nb, about 0.3% or less of Ti, and about 0.01% or less of N; and diameter reduction-rolling the treated steel pipe at a accumulated diameter reduction rate of about 35% or more and a finish rolling temperature of about 500xc2x0 C. to about 900xc2x0 C., the welded steel pipe thereby having a tensile strength of about 780 MPa or more and a nxc3x97r product of an n-value and an r-value of about 0.15 or more.
In this method, preferably, the treated steel pipe is diameter reduction-rolled at a accumulated diameter reduction rate of about 20% or more at a temperature below the Ar3 transformation point.
In this method, preferably, the composition comprises, on the basis of mass percent, about 0.05% to about 0.3% C; about 2.0% or less of Si; more than about 1.5% to about 2.0% Mn; about 0.1% or less of P; about 0.01% or less of S; about 0.1% or less of Cr; about 0.1% or less of Al; about 0.1% or less of Nb; about 0.1% to about 0.3% Ti; and about 0.01% or less of N, wherein the tensile strength of the welded steel pipe is in the range of about 780 MPa to about 980 MPa, and the nxc3x97r product of the n-value and the r-value is about 0.22 or more.
Preferably, in the method, the composition further comprises at least one element group selected from the group consisting of Group A and Group B, on the basis of mass percent, wherein Group A includes at least one element of about 1.0% or less of Cu, about 1.0% or less of Ni, about 1.0% or less of Mo, and about 0.01% or less of B; and Group B includes at least one element of about 0.02% or less of Ca and about 0.02% or less of a rare earth metal.
In the method, alternatively, the composition preferably comprises, on the basis of mass percent, about 0.05% to about 0.3% C; about 2.0% or less of Si; more than about 2.0% to about 5.0% Mn; about 0.1% or less of P; about 0.01% or less of S; about 0.1% or less of Cr; about 0.1% or less of Al; about 0.1% or less of Nb; about 0.1% or less of Ti; and about 0.01% or less of N, wherein the tensile strength of the welded steel pipe exceeds about 980 MPa, and the nxc3x97r product of the n-value and the r-value is about 0.15 or more.
Preferably, such a composition further comprises at least one element group selected from the group consisting of Group A and Group B, on the basis of mass percent, wherein Group A includes at least one element of about 1.0% or less of Cu, about 1.0% or less of Ni, about 1.0% or less of Mo, and about 0.01% or less of B; and Group B includes at least one element of about 0.02% or less of Ca and about 0.02% or less of a rare earth metal.
The welded steel pipe according to the present invention has enhanced formability and particularly excellent hydroformability and high strength and is suitable for use in structural components. This welded steel pipe can be produced by the method according to the present invention at low costs with high productivity.